1. Field of the Invention
The present application relates to the separation of a sheet of brittle material via a laser, and more particularly to laser inducement of a stress fracture within a brittle layer that has a protective coating on its surface, thereby cutting the sheet of brittle material into multiple pieces.
2. Description of Related Art
Flat glass substrates are a suitable substrate material for many applications where transparency, high chemical and thermal resistance, and defined chemical and physical properties are important. These applications typically include those areas where thin film and thick film technologies are utilized including displays, thin and thick film sensors, solar cells, micro-mechanical components, lithographic masks, and electronic applications, such as within sensors and membrane elements. Recent developments in many electronic applications have created a demand for new product functionalities, and have generated an increase in demand for ever thinner and ultra-thin as well as mechanically durable substrates. In particular, these ultra-thin substrates provide light and flexible displays ideal for use within portable pocket devices having a rounded housing form, such as cell phones, pen-type devices with pull-out screens, displays for smart cards, pricing labels, as well as displays that are based on organic or inorganic light emitting layers, or light emitting organic polymer displays (OLED). Thin and durable substrates may also be valued for applications where the final product remains flat, but low cost manufacturing requires mechanically flexible substrates. In a broader category outside of displays, flexible and durable electronic devices in general such as RFIDs, photovoltaics, and sensors value these advanced substrate designs.
The various flexible electronic devices typically have a variety of designs based on their intended application and required performance level. Some degree of bending radius and conformal nature are possible to obtain from liquid crystal displays based on cell gaps as well as OLED or similar displays hermetically sealed between two parallel plates. The larger flexibility displays currently being pursued that are capable of bend radii approaching 1 cm or below are mainly based on OLED, cholesteric liquid crystal, electrophoretic, or similar approaches. These designs typically consist of various transistor circuitries, display, and encapsulating layers built upon a highly mechanically durable substrate.
Substrate materials for manufacturing the individual components typically include thermoplastic materials, such as polyethylene naphthalate, polyethersulfone, polycarbonate, and the like, materials such as metal stainless steel, and glass materials. While the metal substrate materials offer higher thermal capability, they are opaque and may present electrical conductivity issues. The glass material provides the advantages of being chemically and photo-chemically inert, optically isotropic, temperature-resistant, mechanically stable, and provides a relatively hard surface. However, a significant drawback to the glass materials is a tendency to be more brittle and the sensitivity to flaws. The thermoplastic substrates are less sensitive to flaw size, but are limited in their thermal capability as well as their barrier properties. Specifically, limiting water vapor and oxygen permeability of plastic substrate films is difficult, and results in an overall reduction in relative quality and life time of the associated OLEDs or organic electronic devices made on such substrate materials unless a separate barrier layer is used.
As noted above, flexible displays are currently being pursued for future display and electronic technologies. As a result of the drawbacks of singular glass or plastic substrates, glass-plastic composite films are being extensively investigated. Such films typically consist of a thin glass sheet and a polymeric layer applied directly to at least one of the planar surfaces of the glass. Glass sheets in this case refer to sheets of brittle inorganic material that can include glass, ceramics, glass-ceramics, or similar materials sensitive to surface or edge flaws. The polymeric layer refers to any more ductile layer applied to the glass surface for the purpose of protecting against damage. The actual substrate can be made of one or more glass layers with one or more polymeric layers in various configurations. The thickness of any glass layer is typically less than 300 μm for a thin laminated glass substrate for flexible display applications. Laminated or composite glass substrate refers to the combined glass and polymeric structure. The structure can be fabricated through a lamination process of adhering previously formed sheets or through various coating, deposition, curing, or other processes that utilize liquid components.
While such glass-plastic composite films are highly advantageous, applications and performance of such substrates have been severely limited due to low edge strengths caused during the cutting thereof. Heretofore, most glass cutting has been accomplished by mechanical score and break methods. These methods are both simple and economic and can be used for glass sheets with a thickness of a few hundred microns or greater. However, these mechanical cutting techniques, applicable to the thicker sheet glass of approximately 0.5 mm thicknesses or greater, result in a lower yield strength when used to cut glass sheets having a thickness of less than 300 μm. Moreover, complications arise with laminated glass substrates due to the presence of the protective polymeric layer(s).
Efforts to cut these coated, thin glass sheets have thus far included mechanical processes. For example, methods have been disclosed that include heating a plastic layer while simultaneously applying a load via a cutting tool. The plastic is severed simultaneously with the scoring of the glass substrate. This method still relies on a mechanical means of cutting the composite substrate by using physical contact between the substrate and cutting tool, and leads to a rupture of the laminated glass substrate along the scoring line. In the method described in U.S. Pat. No. 6,894,249, entitled METHOD AND DEVICE FOR CUTTING A FLAT WORKPIECE THAT CONSISTS OF A BRITTLE MATERIAL, a laser is utilized to induce a thermal mechanical stress in the work piece along the cutting line. Similar to the method as described above, this method requires a mechanical scoring tool to generate an initial score at the beginning of the associated cutting line. The laser power is then used to produce separation along this cutting line due to thermal stress.
A need exists to efficiently cut substrates composed of multiple materials with widely varying mechanical properties, such as those comprised of glass and polymer. Specifically, these new multilayer substrate designs for applications such as flexible displays and flexible electronics require new cutting methods that are not based on physical contact with a scoring or cutting tool.